In the case of controlling a multiple-winding motor with their respective winding groups connected in parallel to a plurality of inverters, there is an advantage that it is not necessary to provide a reactor for each inverter in order to prevent cross current, as compared to the case of operating a single-winding motor connected in parallel to a plurality of inverters. In addition, in the case of large capacity, it is possible to combine inverters having unit capacities, in accordance with the load capacity, so that there is an advantage that development and manufacturing of inverters become more effective and thus the cost can be reduced.
However, this method causes a problem that, for example, if there is imbalance among currents flowing through the respective winding groups, a large-capacity inverter is needed as compared to the balanced case, or if there is imbalance among the current phases, torque ripple occurs among the windings by interference due to magnetic coupling among the windings.
In order to solve such a problem, the following method is disclosed: the average and the deviation of winding currents are detected, the average value is controlled to be a command value and the deviation is controlled to be zero, thereby equalizing currents flowing through inverters and windings (for example, Patent Document 1).
In the case of using switching devices slow in switching speed, the frequency of the PWM carrier cannot be increased, and therefore a low-order harmonic is left in output voltage. For this problem, low-order harmonic eliminating PWM is disclosed which performs switching at a timing of reducing a specific low-order harmonic, by effectively utilizing a small number of times of switching (for example, Non-Patent Document 1).